This invention is directed to a new and improved method of computing the range to and depth of a submerged object that is capable of either emitting or reflecting transient pulses of acoustic energy and the apparatus for carrying out said method. Although the invention has wide applicability in computing the position of an object within a variety of media, wherein a variety of monitoring apparatus can be utilized, it is particularly suited for utilization as a means for computing the range to and depth of an object submerged within an aquatic environment, as for example, a portion of a vessel, a submerged submarine, a torpedo, a transponder mounted on a diver or other objects whose surveillance is desired, schools of fish or other forms of aquatic life, or the point of detonation of a charge of explosives utilized in a geological survey wherein the monitoring apparatus utilized is sonar.
The need to be able to readily and accurately calculate the range and depth of objects within an aquatic environment has always existed, and has in fact increased over the years, particularly as a result of the advances made in the techniques of undersea warfare. With the increased capabilities of the submarines of today, calculations and the means for making the same to determine range and depth thereof, have taken on a new significance.
Means to measure only the range to a transponder have long been available, although many of the techniques were generally recognized to be rigorously accurate only for idealized situations. None of these techniques, however, provided for the direct and accurate calculation of both the range and depth to a source of acoustic energy, be it active or passive in nature, or enable such calculations under other than idealized circumstances.
It is widely recognized that energy representative of a particular signal pulse emitted from a coherent source rarely arrives at a second receiving or monitoring location by only one path. Generally, the pulse of energy arrives almost, but not quite, simultaneously, by a number of "multipaths" and thus appears to monitoring apparatus, as for example a sonar array, located at the monitoring location, as a number of separate or overlapping pulses of energy that are spaced over a defined interval of time with varying angles of arrival. The nature of these multipaths and the manner by which they are created by the essentially laminar construction of the reflecting surfaces and refracting layers in the ocean are described in standard texts on sound propagation, as for example, "Principles of Underwater Sound For Engineers," by Robert J. Urick, published by McGraw-Hill, Inc., 1967.
The multipaths of energy representative of a single pulse tend to occur in pairs of "doublets." For example, such energy pulses arriving at the receiver from a "down" or bottom direction will travel by two paths which are closely spaced angularly. One path will involve only one reflection from the bottom. The other path will consist of, first, a reflection from the surface, then a reflection from the bottom. Such energy arriving from an "up" or surface direction will also follow two closely spaced paths. One will involve, in order, a surface reflection, a bottom reflection, and a second surface reflection. The other will involve, first, a bottom reflection, then a surface reflection.
Both in the ocean with acoustic energy, and over the ocean with radar pulses, energy will also be received by a direct path and by a reflection (in the acoustic case) off the bottom, and in the radar case, by a reflection off the ocean surface. The direct and one reflection paths constitute a third "doublet" pair.
Through the use of existing methods of sound wave ray tracing techniques, these multipaths can be traced from source to monitoring receiver and from monitoring receiver to source if the vertical angles of arrival formed by the multipaths intersecting the imaginary plane passing through the monitoring receiver and parallel to the surface of the ocean environment are known. By accurately measuring the vertical angle of arrival of at least two multipaths associated with a particular signal pulse, a mathematical computation can be carried out to solve for the range to and depth of the source of the acoustic energy.
Current difficulties in carrying out the above computations arise from a general inability of existing sonars to measure the vertical angles of arrival of each member of the closely associated pairs of paths to sufficient accuracy and/or resolution so as to obtain consistent and reliable computations for range and depth. Sonars available to date can generally measure the arrival angle of a single path to accuracies approaching 0.1 degrees. However, when presented with closely spaced multipaths, whose arrivals at the sonar array overlap in time, as would be the case in operational utilization of the herein disclosed technique, existing sonars indicate the centroid angle of the arriving energy paths which can be as much as 6 to 10 degrees removed from any of the actual arriving multipaths. Although under certain idealized circumstances the above inaccuracies can be minimized and thereby prove acceptable, they generally prove to be too large to be of practical value, particularly in determining the depth of the object which is emitting or reflecting pulses of acoustic energy.
It is, therefore, an object of this invention to devise a new and improved method of measuring the range to and depth of a source of acoustic energy that overcomes the disadvantages of the prior art methods.
It is another object of this invention to devise a method of measuring the range to and the depth of a source of acoustic energy that results in accurate computations for range and depth that is not restricted to idealized sets of conditions.
It is another object of this invention to devise a method of measuring the range to and the depth of a source of acoustic energy that is independent of sonar beam resolution.
It is another object of this invention to devise a method of measuring the range to and the depth of a source of acoustic energy that does not necessarily require measurement of the true angles of arrival of the various multipath arrivals of energy.
It is another object of this invention to devise a method of measuring the range to and the depth of an object submerged within an aquatic environment that can utilize either an active or passive sonar array.
It is another object of this invention to devise apparatus for carrying out each of the above named objects of this invention.
It is another object of this invention to devise a method of measuring the range to and the depth of an object submerged within an aquatic environment that can distinguish between genuine multipath pulses of energy originating from a coherent source of acoustic energy and pulses of energy that are representative of environmental noise.
The objects and advantages of the invention are set forth in part herein and in part will be obvious therefrom, or may be learned by practice with the invention, the same being realized and attained by means of the instrumentalities and combinations pointed out in the appended claims.
The invention consists in the novel parts, constructions, arrangements, combinations and improvements herein shown and described.